Plywood production involves the peeling of a thin continuous layer of veneer from a log, e.g. 0.1 inch thick. The veneer as peeled is a continuous ribbon that is 101 inches wide. It is cut into individual sheets of varying sizes but for the purpose of this invention, the pertinent sizes are half sheets, e.g. 101 inches by 27 inches, and full sheets, e.g. 101 inches by 54 inches. The individual sheets are typically analyzed and stacked by a stacker according to size, grade and moisture content. The sheets are subsequently unstacked, dried and restacked, again by a stacker according to size, grade and moisture content. The acceptable sheets are then made into plywood consisting of laminated sheets of veneer.
It is to be particularly noted that two stacking operations are involved. One is referred to as the stacking of green veneer sheets and the other as the stacking of dried veneer sheets. Whereas there are differences as between the two stacking operations, the improvements provided by the present invention are equally applicable to both of these stacking operations. Hereafter all references to "stacking" unless specifically identified otherwise, has reference to both green and dry veneer stacking operations.
The sheets when distributed to the stacker are separated into designated stacks. For example, one stack may be designated for half sheets having a low moisture content, one for half sheets having a high moisture content, and one stack for moderate or acceptable moisture content. Similar designations of stacks are provided for the full sheets. Other designations are also quite common, e.g. according to grade.
The stacking operation is automatic or in some instances semi-automatic and, as contemplated herein, includes an in-feeding conveyor belt that conveys the individual sheets in sequence to an automatic stacking apparatus. The sheets are analyzed for size, moisture content and grade and then transferred from the incoming conveyor, on which the sheets are bottom supported, to the stacking conveyor, (a plurality of overhead belts) on which the sheets are top supported.
As concerns the overhead or stacking conveyor, air is drawn upwardly through the belts of the conveyor and the suction thus created adheres or attaches the sheets to the overlying surface of the belt. The belt conveys the sheets along the path over the stacking bins which are designated for sheets of specific size and range of moisture content (and where applicable by grade). Knock-off shoes positioned over the conveyor and in line with the bins are activated to dislodge or detach the sheets from the conveyor and deposit them on the stacks in the bins.
The primary consideration of this invention is to deposit the sheets uniformly on the stacks. In particular, the leading edge of the sheet must be carefully deposited to line up with the leading edge of the stack. A number of factors effect this alignment.
If the sheets aren't properly aligned on the overhead belt, they cannot be properly aligned on the stack. Thus the sheets must be properly aligned on the overhead belts.
If the air suction is greater for one sheet than another, i.e. if the suction is not consistent, successive sheets may be released differently and cause misalignment. Thus consistent air suction is desirable.
The left and right or front and back cylinders of the knock-off mechanism can be slightly out of sync and this can cause skewing. Thus the knock-off mechanism needs to be synchronized.
The different weights of the sheets due to moisture variation can change the forward momentum of the sheets as they are transferred to the stack. Thus sheets having different moisture content must be knocked off the conveyor at different positions in order to achieve the desired line up of all the differently weighted sheets within the stack.
All of these problems, in accumulation, typically provide significant misalignment of the sheets in the stack of sheets. Subsequent handling as when a forklift engages the stack, often damages the protruding edge of the misaligned sheets at a very substantial cost to the producer.